In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter.
Especially thermal plates, which are sensitive to heat or infrared light, are widely used in CtP methods because of their daylight stability. Such thermal materials may be exposed directly to heat, e.g. by means of a thermal head, but preferably comprise a compound that converts absorbed light into heat and are therefore suitable for exposure by lasers, especially infrared laser diodes. The heat, which is generated on image-wise exposure, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained. Many thermal plate materials are based on heat-induced ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the plate-setter. As a result, such ablative plates require a processing step for removing the debris from the exposed material.
EP-A 770 494 discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby inducing coalescence of the polymer particles and converting the exposed areas into an hydrophobic phase which defines the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is developed by interaction with the fountain and ink that are supplied to the cylinder during the press run. During the first ten or twenty revolutions of the press, the non-exposed areas are removed from the support and thereby define the non-printing areas of the plate. So the wet chemical processing of these materials is ‘hidden’ to the user and accomplished during the start-up of the printing press. Other prior art documents such as EP-A 770 497 and U.S. Pat. No. 6,001,536 describe the (off-press) development of similar materials with water or an aqueous liquid.
Until now, heat-induced polymer particle coalescence is the only heat-triggered non-ablative imaging mechanism that requires no separate processing step with alkaline chemicals and that meets all the requirements for making a high-quality printing plate material (Agfa Thermolite®). Various improvements of such materials are described in e.g. EP-As 773 112; 774 364; 802 457; 816 070; 849 090; 849 091; 881 095; and 931 647. However, none of the prior art materials, which work according to heat-induced polymer particle coalescence, is suitable for making printing plates that provide a high run length during printing. Degradation of the print quality due to image wear limits the run length to a maximum of typically 25,000 printed copies. Also the limited mechanical robustness (scratch sensitivity) and chemical resistance towards press chemicals such as plate cleaners, blanket cleaners and fountain additives contribute to the mentioned low printing endurance of such plates.